1. Field of the Invention
The present invention relates to a carburized hardening process for a power transmission member such as gears, in which teeth are formed at equal intervals on the peripherary with a groove therebetween, to which side high surface pressure is applied, and to which root (edge portion) high bending stress is applied.
2. Description of Related Art
For gears such as pinions of automotive differential gears, in general, case-hardened steel is used, on the surface of which is gas carburized hardened or plasma carburized hardened for improving fatigue strength and wear resistance before application. However, because in gas carburized hardening and plasma carburized hardening generally practiced, the work is heated at high temperature for a long time, impurities such as phosphorus (P) or sulfur (S) segregates at the grain boundary or carbide precipitates in a mesh-form to lower the strength of the grain boundary.
Consequently, crack is likely to be generated at the edge portion of the root subjected to high bending stress (shown with A in FIG. 1) with the grain boundary as an initiation point, and cracks quickly propagate, resulting in a problem of short breaking strength. In the tooth side subjected to high surface pressure (shown with B in FIG. 1), there was also a problem of short spalling resistance.